The temporoparietal junction (TPJ) is an area of the brain where the temporal and parietal lobes meet, at the posterior end of the lateral sulcus (Sylvian fissure). The TPJ incorporates information from the thalamus and the limbic system as well as from the visual, auditory, and somatosensory systems. The TPJ also integrates information from both the external environment as well as from within the body. The TPJ is responsible for collecting all of this information and then processing it.
This area is also known to play a crucial role in social cognition, including perspective-taking, empathy, self–other distinctions processes and theory of mind (ToM). The TPJ is also involved in attentional reorientation, enabling individuals to shift attention toward relevant environmental stimuli. Furthermore, damage to the TPJ has been implicated in having adverse effects on an individual's ability to make moral decisions and has been known to produce out-of-body experiences (OBEs). Electromagnetic stimulation of the TPJ can also cause these effects. Apart from these diverse roles that the TPJ plays, it is also known for its involvement in a variety of widespread disorders including anxiety disorders, amnesia, Alzheimer's disease, autism spectrum disorder, and schizophrenia.
Anatomy and function
The brain contains four main lobes: temporal lobe, parietal lobe, frontal lobe, and the occipital lobe. The temporoparietal junction lies in the region between the temporal and parietal lobes, near the lateral sulcus (Sylvian fissure). Specifically, it is composed of the inferior parietal lobule and the caudal parts of the superior temporal sulcus. There are two halves to the temporoparietal junction, with each component in their respective hemispheres of the brain. Each half of the TPJ pertains to various aspects of cognitive function. Often, however, the separate halves of the TPJ will work in coordination. The TPJ is mainly involved in information processing and perception.
Right temporoparietal junction The right temporoparietal junction (rTPJ) is involved in the processing of information in terms of the ability of an individual to orient attention to new stimuli. Evidence from neuroimaging studies as well as lesion studies revealed that the rTPJ plays a pivotal role in analyzing signals from self-produced actions as well as with signals from the external environment. For example, an individual with lesions in their rTPJ would more than likely exhibit a sense of hemi-neglect, wherein they would no longer be able to pay attention to anything they observe on the left. So, if someone were to have a lesion in their rTPJ, then over time the awareness of the left limbs may fade without treatment. Visual signals provide the sensory information necessary for the brain to process spatial recognition of the world. When vision is limited, knowledge of existence begins to fade away since as far as the brain is concerned the object does not exist. Furthermore, the rTPJ plays a role in the way individuals observe and process information, thus impacting social interaction. Empathy and sympathy require an individual to simultaneously distinguish between different possible perspectives on the same situation. Imaging studies show that this ability depends upon the coordinated interaction of the rTPJ to identify and process the social cues presented to it. This rapid process allows for an individual to quickly react to situations.
Left temporoparietal junction The left temporoparietal junction (lTPJ) contains both Wernicke's area and the angular gyrus, both prominent anatomical structures of the brain that are involved in language cognition, processing, and comprehension of both written and spoken language. Steven Pinker discusses this brain region, theorising that it underlies an amodal 'language of thought' or Mentalese. The lTPJ, in this account, takes in observations from external environments, such as conversations, makes connections in the brain regarding memories or incidents and then converts those thoughts and connections to written and spoken language. Pinker's full account of this is explained in The Language Instinct: How the Mind Creates Language. The lTPJ also plays an important role in reasoning of other's beliefs, intentions, and desires. Activation of the lTPJ was observed in patients processing mental states such as beliefs when an fMRI was used on patients as they were asked to make inferences regarding the mental states of others such as lying. This study was further supplemented by a study which identified that lesions to the left TPJ can impair cognitive processes specifically involved in the inference of someone else's belief, intention, or desire. Individuals with lesions in the lTPJ were no longer able to correctly identify when someone was lying or insinuating a false sense of belief or desire. The lTPJ is also involved in the processing of associating and remembering the names of individuals and objects.
Disorders The dopaminergic-serotonergic system mediates our ability to distinguish and understand others' beliefs as well as predict their behavior in light of that understanding. In certain disorders involving the dopaminergic-serotonergic system, this mentalizing process is disrupted and part or all of the process is impaired; this includes amnesia, Alzheimer's disease, and schizophrenia.
Amnesia Amnesia is a deficit in memory caused by brain damage, disease, or physiological trauma. Amnesia is best understood via Henry Molaison, or patient H.M., who suffered from severe epilepsy and eventually had a temporal lobectomy. After surgery, his epilepsy improved but then he had anterograde amnesia, wherein long-term memory formation is inhibited. Short-term memory remained normal except that he could never remember anything that had happened after his surgery for very long. Based on general known roles of the TPJ, it is known that the TPJ is involved in the memory processing system of the body. Studies have also revealed that certain types of epileptic amnesia could be attributed to TPJ. fMRI studies indicated that there was lower activation of the rTPJ in patients with epileptic amnesia. Furthermore, it was noticed the autobiographical memories were affected in these patients. As such, the rTPJ along with the right cerebellum were identified as core components of autobiographical memory.
In terms of treatment, most forms of amnesia fix themselves without actually undergoing treatment. However, options such as cognitive therapy or occupational therapy have proved to help. Therapy will focus on various methods to improve a patient's memory and with repetition over time, a patient's memory as a whole will improve and eventually become close to normal.Mayo Clinic Staff (2011) Amnesia: Treatments and Drugs. Mayo Clinic. Retrieved from: http://www.mayoclinic.com/health/amnesia/DS01041/DSECTION=treatments-and-drugs.
Alzheimer's disease Alzheimer's disease is the most common form of dementia and is also the sixth leading cause of death in the United States. This disease has no known cure and is a disease that worsens as it progresses and eventually leads to death. Reduced metabolism in the TPJ, along with the superior frontal sulcus, correlates with Alzheimer's patients' inability to perceive themselves as others do (with a third-person point of view); the discrepancy between a patient's understanding of their own cognitive impairment and the actual extent of their cognitive impairment increases as metabolism in the TPJ decreases. Additionally, the TPJ contains the praxicon, a dictionary of representations of different human actions, which is necessary to distinguishing between actions of the self and other people. Because dementia (including Alzheimer's) patients with anosognosia are unable to distinguish between the normal actions of other people and their own diminished abilities, it is expected that damage to the TPJ is arresting this cognitive function.
Autism spectrum disorder There may be a connection between the temporoparietal junction and how individuals with autism spectrum disorder's recognition of socially awkward situations may differ from those not autistic. Research reported in 2015 from an experiment in which participants, high-functioning adults with autism spectrum disorder (ASD) and non autistic controls, were asked to watch socially awkward situations (a complete episode of the sitcom The Office) under an fMRI, which measured their brain activity. Several brain regions implicated in social perceptual and cognitive processes were of interest: "the dorsal, middle and ventral parts of medial prefrontal cortex (DMPFC, MMPFC and VMPFC), right and left temporo-parietal junctions (RTPJ and LTPJ), right superior temporal sulcus (RSTS) and temporal pole, and posterior medial cortices [posterior cingulate, precuneus (PC)]." In general, participants' activity in several of those brain regions tracked the episode's socially awkward moments to similar extents—the results were evidence of a of group difference : their activity near the RTPJ, spanning into the posterior end of the RSTS, showed notable quantitative differences between the ASD and NT groups (with ASD group showing lower activity).
Research reported in 2016 on ASC-related structural or physiological differences found using neuroimaging noted that results are often inconsistent across the literature, which could be caused by a variety of variance sources. (Re-)analysis using a technique they developed to reduce one common external source of variance showed group differences in TPJ. However, although statistically significant, results did not display the discriminative power sufficient to classify diagnostic groups, instead yielding accuracy results close to random. They concluded that ASD is a highly heterogeneous syndrome/diagnostic category whose differences from NT controls are difficult to characterize globally using neuroimaging.
Schizophrenia The decreased ability for schizophrenia patients to function in social situations has been related to a deficit within the theory of mind process. There have been relatively few studies that have examined the role of theory of mind in schizophrenia patients; the findings of these studies as they relate to the activation of the TPJ are varied. Some studies have found decreased activation of the TPJ in schizophrenia patients who were asked to make inferences about other peoples' social intentions based on cartoons; other studies, however, performed similar assessments of schizophrenia patients and found that the TPJ actually became hyperactive, compared to control individuals without schizophrenia, in the TPJ. This indicates that there is abnormal activation of the TPJ in these patients while performing tasks that involving understanding social intention of others, but the directionality of this abnormal activity is not clear, or possibly not universal throughout schizophrenia patients. It was found that the changes in activation in the TPJ were lateralized; they found that there was reduced activity in only the right TPJ and proposed that based on previous research about the different roles of the right and left TPJ the findings indicated that there was a more general deficit in the overall mentalizing process for these patients, but their ability to understand other individuals' basic social intentions through observing interaction is not impaired.